Photovoltaic (PV) solar cells are used more extensively every day as an alternative energy source. Typically, a PV solar cell is used to provide solar power for a multitude of applications, both terrestrial and space. The more widespread of the two applications, terrestrial, mainly consists of solar powered consumer products, such as calculators, remote power applications, such as remote telecommunications power supplies, and utility generation applications, such as home electricity generation. Space solar power applications, on the other hand, mainly consist of solar power for space satellites.
As stated above, solar cells are employed by the terrestrial and space industries to provide solar power for numerous applications. Conventional PV solar cells create energy by converting sunlight directly into direct current (DC) electricity, and typically consist of a P-N or N-P junction semiconductor such as single-crystal silicon (Si), gallium arsenide (GaAs) or a gallium arsenide/germanium (GaAs/Ge) class of cells with a dual-junction or triple-junction arrangement. When PV cells are exposed to sunlight, the cell absorbs a portion of the light. The absorbed light excites electrons within the p-type region of the semiconductor, and those electrons flow into the n-type region. The electrons build up a negative charge in the n-type region and, through a load connecting the p-type and n-type regions, flow back to the p-type region. The flow of electrons across the load creates an electric current and, thus, power across the load.
Most conventional PV cells use a top/bottom contact arrangement whereby the current from a front surface of the cell is channeled, via metal grids, to a front contact while separate contacts are placed on a rear surface to access the rear polarity. Referring to FIG. 1, some PV cells 10 consist of a coplanar arrangement whereby a contact from the front surface 20 is wrapped around or through the cell to the rear surface so that both the positive 40 and negative 60 contacts are on the rear surface 30. In coplanar PV cells having a wrapped around configuration, an insulating, dielectric material 50, is typically wrapped around the PV cell between the cell and the wrapped-around contact to isolate the contact from the remainder of the PV cell. Whereas PV cells consist of a variety of different material arrangements, coplanar arrangements are typically only available on PV cells made from Si.
Due to the energy demands of many applications, a single PV cell generally cannot generate the desired amount of voltage or current. Therefore, to achieve a desired voltage output, multiple PV cells are electrically connected in series to form strings. These strings are then arranged in parallel configurations called arrays to achieve desired current levels for particular applications. To electrically connect the PV cells, the cells can be attached to a flexible circuit. Each PV cell is then covered by a respective glass cover plate to protect the PV cell from the environment. For example, U.S. Pat. No. 4,133,697 issued Jan. 9, 1979 to Mueller et al. (hereinafter “the '697 patent”) discloses a flexible solar array strip employing printed circuitry sandwiched between a pair of layers of a polyimide material. As disclosed by the '697 patent, the solar cells are interconnected through the printed circuitry as a result of solder pads extending through apertures formed in one of the polyimide layers so as to connect the contacts on the solar cells with portions of the printed circuitry.
Generally, the same type of configurations can be used for both terrestrial and space PV cell applications. But due to differences between environments, PV cells used for space applications generally require a more precise and, thus, costly manufacturing process. Terrestrial applications have begun to use less costly alternatives to single-crystal Si, including poly-crystalline silicon and a variety of thin film PV cells, such as amorphous silicon thin film PV cells. But these alternatives have much lower efficiencies than Si or GaAs, generally making them impractical for space PV cells. In this same vein, the PV cells for space applications desirably have very small spacing between adjacent cells, such as about 0.030 inches, in order to capture a greater percentage of the incident light. In contrast, PV cells for terrestrial applications generally have greater spacing between adjacent cells so as not to require as precise of a manufacturing process. Additionally, the PV cells used for space applications must also be more reliable and longer lasting than the PV cells used for terrestrial applications since space-deployed PV cells cannot be replaced.
In addition to PV cell efficiency and reliability, space applications using PV cells must also address an array's weight and exposure to the space vacuum, radiation and plasma environments that also typically increase the required precision and cost of manufacturing. Due to cost and weight factors, space PV cells generally require very thin glass plates, typically on the order of 0.008 inches thick, whereas terrestrial PV arrays typically operate with a glass plate one-eighth of an inch thick. Typically, space PV arrays are hand-assembled and generally have non-uniform spacing between cells. And because space PV arrays typically use cells with front/rear surface contacts, space PV arrays typically require each cell to have its own glass cover plate. As such, the overall manufacturing process is slowed in order to separately place a glass cover plate on each cell. Additionally, hand assembly of space PV arrays typically decreases manufacturing precision. In this regard, to raise the amount of power generated over a given illumination area, it would be desirable to minimize the spacing between adjacent cells to about five thousandths of an inch, thereby requiring a level of precision in fabricating the tiles not generally found in hand assembly. Also, the manufacturing of space PV arrays with separate cover plates typically uses alignment pins to more accurately place each cell within a string. The use of such pins, however, can lead to the damaging of the PV cells if the pins are misaligned relative to the cells or inaccurately moved in relation to the cells. Thus, there exists a need for a space PV array that can use a single cover plate, and an associated fabrication method that is faster and more precise than conventional methods.
Not only do individual cover plates increase manufacturing cost for conventional space PV arrays, but they also typically result in both the cell edges and cell-to-cell interconnects being exposed to space plasma. To decrease potential damage caused by plasma charging, space PV array voltages are frequently maintained below 50 volts and/or all exposed interconnects are conformal coated with a dielectric material. While mitigating damage to the cells, the lower voltage results in a significant harness loss for higher power applications, and conformal coating adds additional integration cost and mass to the array. Thus, there exists a need for a space PV array that decreases the potential damage caused by plasma radiation and can safely operate above 50 volts without requiring conformal coating.